1. Field of the Invention
The present invention relates to semiconductor electronics and, more particularly, to photovoltaic energetics, and may be used for providing photovoltaic cells.
Devices for converting light energy to electric energy (photoelectric cells) are known, which comprise a barrier-structure semiconductor wafer, conductive current collecting elements arranged on both sides of the wafer, protective coatings as well as current tapping electrodes. Current collecting contacts are one of the construction elements of photovoltaic cells.
2. State of the Art
Present-day photovoltaic cells include current collecting contacts deposited by vacuum evaporation through a mask (cf. A. Fahrenbruch, R. Bube "Solar Elements. Theory and Experiment", Moscow, Energoatomizdat, 1987, p. 169).
The following metals are deposited using vacuum evaporation techniques: aluminum, titanium, palladium, copper, and silver. A copper or silver layer is deposited as the top layer. Following deposition the contacts are coated with lead- and tin-based solder. Thus, the use of vacuum-deposited contacts in photovoltaic cells requires significant amounts of expensive metals and energy.
In U.S. Pat. No. 4,235,644, photovoltaic cells are disclosed, wherein the current collecting contacts are made using a screen-printing technique. A conductive paste containing silver powder as a filler is applied by means of a roller through a mask to the semiconductor wafer surface. After removal of the mask the contacts are fired.
A major drawback of photovoltaic cells having vacuum-deposited or screen-printed contacts is that, in practice, it is not possible to accomplish contact buses less than 0.2 mm in thickness.
The mesh of such contacts deposited on the surface of the semiconductor wafer makes it impossible to provide a surface shadowing area of less than 5 percent of the total area, which is a limiting factor to improving the conversion efficiency of photovoltaic cells. Moreover, when photovoltaic cells containing vacuum-deposited or screen-printed current collecting contacts are used to make batteries, the interconnection of the photovoltaic cells is a complicated and time-consuming task to be performed either manually or using expensive sophisticated equipment.
In U.S. Pat. No. 4,380,112, a photovoltaic cell is disclosed, which comprises a silicon semiconductor wafer having a barrier structure, a continuous metallic contact on the rear side, and a current collecting contact on the front side. The current collecting contact on the front side (i.e. the side oriented towards incident light) is a metallic wire embedded in borosilicate glass and directly contacting the surface of the semiconductor wafer. Since the glass is electrostatically bonded to the surface of the semiconductor wafer and the whole construction is heated to a temperature at which a noticeable diffusion of the wire metal into the semiconductor takes place, a rigid contact is provided between the semiconductor wafer and the current collecting contact.
The disadvantage of such a construction, too, is low conversion efficiency due to a considerable degree of shadowing of the semiconductor wafer surface caused by the wire. Here, too, the shadowing area constitutes at least 5 percent of the total surface area of the semiconductor wafer. Furthermore, this construction is more expensive than the two constructions described above, as borosilicate glass is used, an expensive material which satisfies the requirements of matching the thermomechanical properties of the silicon wafer and the glass cover plate. This construction requires preliminary polishing of the semiconductor wafer and the glass plates and, therefore, cannot be used for constructions in which inexpensive polycrystalline or rough monocrystalline silicon is employed. Elevated temperatures up to 600.degree. C. are required to achieve electrostatic bonding.
Moreover, this construction requires subsequent sealing of the whole photovoltaic cell with a sealant polymer and glass cover plates. Photovoltaic cells of this type are usually interconnected by connecting the wires on the front side and the metallized contacts on the rear side by soldering. This process is rather difficult to automate.